Vented batteries emit potentially explosive gases under charge conditions. Therefore it is important to calculate the amount of ventilation required for your battery compartment. The required number of air changes per hour (A) is given by the following formula:

A = (0.045 x N x I) / V

Where:
N = Number of cells in the battery
V = Volume of compartment in cubic metres
I = Charge rate in Amperes

This formula will give the number of air changes per hour required during bulk charge conditions. On float charge, the amount of gas emitted is approximately 1.5% of that liberated whilst on boost charge and under most circumstances this will be dissipated by natural ventilation, and will not present a hazard. However, it’s recommended that the bulk charge condition is allowed for at the design stage to ensure the appropriate decision on ventilation requirements is made. Although Valve Regulated Lead-Acid Batteries require little ventilation under normal operating conditions, it is good practice to apply the formula to calculate the number of air changes required to achieve minimum risk under battery fault or failure conditions.

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Gord May "If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

I am project coordinator for a Consulting Engineering firm that designs (Elect/Mech/Struct) building systems. The formula derives from ventilation requirements for Battery Charging Rooms, as used for Commercial/Industrial applications (Stand-By battery systems, Elect. Fork Trucks, etc). I only post the formula to illuminate the importance of (at least 'some') ventilation (for gas removal, not cooling).
Of course the exhaust must be discharged ‘outside’, in a similar manner to the enginebilgeexhaust. After all, you don’t want to disperse the potentially explosive gas inside the boat.

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Gord May "If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

GordMay Once Whispered In The Wind
Of course the exhaust must be discharged ‘outside’, in a similar manner to the enginebilge exhaust. After all, you don’t want to disperse the potentially explosive gas inside the boat.

Yeah!!

I can imagine a boat owner having some elderly guest over when those batteries blows. KAAABOOOOM!!!

Talking about a mutiny. Or some people happening to need to go to hospital over a heartattack?

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CaptainK
BMYC

"Those who desire to give up Freedom in order to gain security, will not have, nor do they deserve, either one." - Benjamin Franklin

Those formulas that Gord showed us were developed years ago when industrial chargers had little or no regulation much less multiple-step charging. The formulas are fairly conservative for a safety factor when heavily gassing flooded-cell batteries.

Such chargers will quickly ruin Valve-regluated sealed batteries and no enclosed vessel should use such chargers for the reasons Gord mentions.

Multi-step chargers are a "first-approximation" attempt to produce product that emulates "Amp-hour-law" charging. Such charging requires a battery monitor controlled regulator that delivers a charge current equal to the value of Amp-hours "missing" from the battery. By following such a method a battery can be completely recharged in a minimum time without excessive gassing or temperature rise.

Because a properly set-up multiple step charger (like the old Link 1000 or Link 2000 controlling a Heart inverter/charger....now Xantrex) you will notice that in the bulk mode there will be essentially no gassing because almost all of the charge energy is maximally "spent" in forcing the chemical reversal between lead sulphate into lead oxide and water into sulpheric acid. With a true Amp-hour law charger you will notice almost no gassing from dead to full.

So on the typical cruiser, with a bank of batteries under the quarterberth, how likely are we to have a problem ? Cause ? Will closed cell such as AGM avoid the risk ? Does venting out mean a powered fan system venting to cockpit similar to engine vent ?

AGM and gel-cel valve regulated batteries should never vent to the outside of their cases. If they do then the charger is not "set" to the correct voltage (or current). Should they vent water will be lost from the internal electrolyte raising the standing voltage (bad) because the standing voltage is directly proportional to the specific gravity of the electrolyte: loose water and the specific gravity increases, increase the specific gravity and you begin to prematurely erode away the internal lead (and other bad things happen to limit the life).

I have routinely charged good AGM and gel-cell batteries at very high charge rates without causing venting of the cases. The key is to have a "real" battery monitor to tell you just how much current you can charge (The number of Amp-hours missing is the value of charge Amps that can be safely used).

In addition, once an AGM or Gel-cell battery has completely recovered lost energy and is full and is not hot (not much over 80 deg F) the terminal voltage can be raised to 16V and the battery will just not charge accept. I do this as one indication that the battery is truly "full". If it charge accepts much more than 100mA per 100A-h rating at 15 or 16V it is not full yet or has not has its capacity recovered from previous cycling.

No, you will not get this info from the battery distributors.
Obvously the best cruising solution is to have a battery monitor controlling the charger in the "Amp-hour-law" mode. So far none exist on the market and you have to do it somewhat manually if you have voltage control of your charger.

Valve regulated sealed lead-acid batteries DO gas internally when charged. When charging properly the gassing rate is equal to or less than the maximum recombination rate internally and, although internal pressure raises (only a psi or so) it does not reach a value that causes the valves to vent to the outside.

Gord raises a safety question of, "What if'? Shoud some catastrophic failure occur in the regulation of a charge source do you have a way to deal with the venting?

I have 4 Trojan T-105's (flooded) batteries, for a total of 900AH. I have 2 Iota 90 amp "smart" chargers with each one charging a set of the Trojans. (Each chargers "sees" 450 ah of battery)

I use at MAX 250 ah per day, so that's what I'm replacing... but I'm replacing it pretty quickly since I have these powerful chargers running off a genset.

Do I need to worry about gassing? I'm thinking not, based on Rick's response, but I wanted to make doubly sure.

My plan... if I do need to worry about gassing, is to simply install a grate, as my batteries are below the cabin sole in the main salon. Let the H2 rise up and right out the hatches if this is the case. If I don't need to worry about gassing... no vents at all, like the current setup.

Since the T105's are 6V, and I presume your system is 12V, then your capacity is 450Ah instead of 900. As for gassing, I have the T105's and when they are charging hard through the 120A Prosine charger, I get just a few small bubbles of gas lazily rising to the surface of each cell. Certainly not a lot of gassing. Mine are not actively vented and are under a settee where any gassing is relieved through a passive vent. This is the factory setup for the boat (although that doesn't mean it's correct). Hydrogen quickly disperses and is only combustible in concentrated quantities. As long as the batteries vent into a large space like your salon, you won't need to worry about the big boom.

I wouldn’t insist on installing either a Battery Compartment Exhaust Fan, nor Natural Ventilation Louvres (cabin sole grate), in your application. Your maximum ventilation requirement might be* something on the order of 20 - 35 cfm, while charging at maximum rate (*except see Rick’s earlier advice).
By way of comparison, a small 3" Bilge Blower would be capable of venting about 100 cfm, in a typical installation.

Notwithstanding:

ABYC E-10.7.10 states:
A vent system, or other means, shall be provided to permit the discharge from the boat of hydrogen gas released by the battery.

Hydrogen gas is colorless, highly flammable*, very light**, cannot sustain life***, and reacts easily with other chemical substances.
* Flammability limits in air: 4.0% -to- 75% by volume - a huge range!
(most standards limit accumulation to 2% by vol.)
** Specific gravity: 0.0696 @ 70 deg. F (much lighter than air @ S.G. 1.0)
*** Non-toxic; but may cause death by suffocation by diluting the concentration of oxygen in air below levels necessary to support life.

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Gord May "If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

Thanks Mark and Gord. I will go un-vented. It looks better that way anyway.

Question: I'm not sure what I'm missing regarding the Trojan T-105's. I did a lot of research. I read that the 6V T105's are each rated at 225ah. I assumed that they retained that rated when I hooked them up in series to produe 12V. This isn't true?

Could be the 14 hour days I've been having lately. Too exhausted to figure it out right now.

Since the T105's are 6V, and I presume your system is 12V, then your capacity is 450Ah instead of 900. As for gassing, I have the T105's and when they are charging hard through the 120A Prosine charger, I get just a few small bubbles of gas lazily rising to the surface of each cell. Certainly not a lot of gassing. Mine are not actively vented and are under a settee where any gassing is relieved through a passive vent. This is the factory setup for the boat (although that doesn't mean it's correct). Hydrogen quickly disperses and is only combustible in concentrated quantities. As long as the batteries vent into a large space like your salon, you won't need to worry about the big boom.

I had 4 Trojan's venting in a settee for a long time next to the AC unit. It was eating away at the aluminum exchanger. They do need venting. Explosions aside the gas is at least corrosive. It would be nice if you could have it go some place out side or at least not near things that it will eat. It's part of why I do do floods any more.